1. Field of the Invention
The present invention relates to a method and an apparatus for ascertaining the properties of the battery, in particular of a traction battery.
2. Description of the Prior Art
It is clear that in future, both in stationary applications (such as in wind farms) and in vehicles (such as hybrid and electric vehicles), new battery systems will increasingly come into use. The terms “battery” and “battery system” are used in the present specification, adapted to conventional linguistic usage, for “accumulator” and “accumulator system”.
The basic functional layout of a battery system of the prior art is shown in FIG. 2. To attain the required power and energy data with the battery system, in a battery individual battery cells 1a are connected in series and in part additionally in parallel. Between the battery cells 1a and the poles of the battery system is a so-called safety and fuse unit 11, which for instance connects the battery 1 into external systems and undoes that connection, protects the battery system against impermissibly high currents and voltages, and also performs such safety functions as the single-pole disconnection of the battery cells 1a from the battery system poles if the battery system housing is opened. A further function unit is formed by the battery management 12, which besides the battery state identification 12a also performs the communication with other systems as well as the thermal management of the battery 1.
The function unit for battery state identification 12a shown in FIG. 2 has the task of determining the actual state of the battery 1 and of predicting the future performance of the battery 1, for instance predicting its life and/or range. Predicting the future performance is also called forecasting. The basic layout of a model-based battery state identification is shown in FIG. 3. The model-based battery state identification and prediction shown is based for instance on evaluating the electrical variables of the battery current, the battery voltage, and the temperature of the battery. The battery state identification can be done for individual cells 1a of a battery 1. This is then done on the basis of the corresponding cell voltage, cell current, and cell temperature. The battery state identification can also be done for the entire battery 1. This is then done—depending on the requirements for precision—either by evaluating the states of the individual cells 1a of the battery 1 and an aggregation based thereon for the entire battery 1, or directly by evaluating the total battery voltage, battery current, and battery temperature. One of the essential items of information that describe the aging state of the battery cells 1a is the reduction in capacity of the cells 1a over the life. All the present methods for ascertaining the capacity of the cells 1a share the feature that the courses of current, voltage and temperature that occur in normal operation of the battery 1 are used for ascertaining the capacity. To that end, in normal operation of the cells, changes in the charge state of at least 20% must occur, in order to attain satisfactory precision in ascertaining the capacity. Moreover, the charge that can be withdrawn in total from the battery cells 1a is very strongly dependent on the magnitude of the discharging current. Precise determination of the capacity of the battery cells 1a, which relates to standard discharge conditions at room temperature and a discharging current, for instance of 1C (discharging current in A corresponds to rated capacity of the battery in Ah) during normal operation of the battery 1 is therefore quite difficult, since arbitrary current courses can occur, with different high discharging currents and with charging phases occurring in between.
In FIG. 4, the functional principle of an arrangement for so-called resistive balancing of battery cells is shown. The object of cell balancing is, in a series circuit of a plurality of individual cells 1a, to ensure that the cells 1a all have nearly the same charge state or nearly the same cell voltage. Because of the fundamentally existing asymmetries of the battery cells 1a, such as slightly different capacitance or slightly different self-discharging, ensuring that they have virtually the same charge state or cell voltage could not be done during operation of the battery 1 unless additional provisions are made. In resistive cell balancing, the battery cells 1a can be discharged by way of connecting in an ohmic resistor RBal—n, disposed parallel to the cell, by connecting the resistor RBal—n in parallel to the cell n via the transistor TBal—n. By discharging those cells 1a that have a higher charge state or a higher voltage than the cell or cells 1a with the least charge state or the least voltage, the charge states or voltages can be made symmetrical throughout all the cells 1a of the battery 1. Triggering the transistor TBal—n is done via an associated control and evaluation unit SW, which taps the cell voltage via a filter Fn and an analog/digital converter ADn. In lithium-ion batteries, which comprise a series circuit of a plurality of individual cells, the state of the art is to use resistive cell balancing. There are still other methods for cell balancing that can operate virtually without loss, such as so-called inductive cell balancing, in which reloading energy stored in individual cells 1a is done via an inductive resistor, for instance. A basic illustration of inductive cell balancing is shown in FIG. 5.
The model-based battery state identification and prediction presented above is based on evaluating the electrical variables of battery current and voltage as well as the temperature of the battery. All the methods in the prior art have in common the fact that the courses of battery current, voltage and temperature in normal operation of the battery are used for ascertaining the battery state and for predicting future performance. In vehicles, the battery state identification operates automatically.